Prospects of flora and vegetation reconstructions dependent on sediment petrological conditions, exemplified by eight Egerian floras of Hungary

Sediments of fossiliferous layers of eight Late Oligocene (Egerian) fossil plant sites located in the Hungarian Egerian basin were analyzed by grain-size distribution (wet sieving and laser grain-size distribution). Based on the previous flora and vegetation reconstructions the region defined by the site studied was covered at that time by relatively continuous and uniform zonal vegetation composed of paleotropical elements. Thus, the selected localities differ from one another only with regard to their stratigraphic setting, i.e. whether the fossils were obtained from deeper, more clayey or shallower, more sandy deposits of the oscillation sequence characteristic of the Egerian deposits. At least 50% of the sediment must belong to the mudstone fraction (<0.063 mm) if the majority of plant species are undoubtedly identifiable, while 70% or more, would be optimal, for which the site at Pomáz is a good example. The carbonate content of fossiliferous sediments significantly reduces the chance of good preservation and thus increases the number of unidentifiable taxa. Based on the comparison of plant fossil taxonomical studies, the grain-size distribution and the mineralogical composition of the fossiliferous sediments, a correlation was pointed out what conditions would be required by each taxon to yield identifiable specimens in the fossil record. According to the results of grain-size distribution the present study gives an opportunity—in the course of floristic studies and vegetation reconstructions—to develop a model reflecting the chance of the occurrence of each fossil taxon.